1. Field of the Invention
The present invention relates to filter systems, and more particularly, to a method and apparatus of providing an undistorted voltage reference signal to a power factor correction circuit that is implemented in an electromagnetic interference (EMI) filter system.
2. Description of the Related Art
Over the past few decades, commercial aircraft has become a necessary mode of travel for personal and business reasons. To improve passenger comfort, commercial aircraft typically offer a number of entertainment activities that can be enjoyed by the passenger, such as video games, movies and various selections of music. In addition, various electronic systems for supporting communications and/or other in-flight functions are provided.
On such commercial aircraft, there are typically a large number of electronic units having power supplies that are coupled to alternating current (AC) power sources. Power line distortion emitted from components in the power supply, typically termed conducted emissions, contribute to the appearance of a distorted load to the AC power source. Such conducted emissions, which are typically generated by the electronic components within the power supply or within circuits in the electronic units, can adversely affect the AC power source and other equipment sharing the AC power source. To minimize such conducted emissions, electronic units that are coupled to an aircraft's AC power source must undergo qualification testing having stringent standards. The limits of conducted emissions that are typically tested include: broadband and narrowband conducted current limits, harmonic current limits and minimum power factor requirements. The harmonic current limits and minimum power factor requirements apply directly to the performance of the power supply. The broadband and narrowband conducted current limits apply to the measurement of current in the AC power lines connected to an electronic unit and may not be related to the power supply itself.
To further minimize the effects of conducted emissions, a power line filter, such as an EMI filter, is typically incorporated within the power supply to suppress the electronic emissions that would otherwise be coupled on to the power lines. Units that are equipped with active power factor correction circuitry control the flow of power to the unit with the ultimate design goal of making the unit simulate a resistive load; that is, a load in which the current drawn from a sinusoidal voltage source is also an undistorted sinusoid and in phase with the source.
In typical commercial aircraft applications, it is necessary to use power factor correction to meet the power factor and harmonic current limits. One form of power factor correction involves the use of passive filters to attenuate the higher order harmonics of the power supply. For single phase power applications, these filters are too bulky and heavy to be used extensively on an aircraft. The preferred method for single phase systems is to use active power factor correction circuitry. Such an active, or electronically controlled approach, usually involves the implementation of a power factor correction (PFC) integrated circuit (IC) and supporting components to create a boost switching power converter topology which converts the rectified AC input voltage to a high DC output voltage, which can then be filtered and regulated down to voltages needed to power the electronic circuitry. The active PFC circuit regulates the load voltage by applying current to the load in phase with a reference voltage which is typically the output of the main bridge rectifier.
In such a conventional approach, the AC reference voltage is measured from the output of the main power rectifier, which is positioned on the load side of the EMI filter, as shown in FIG. 1. Such an approach tends to generate a distorted reference signal, because the high amplitude currents in the main rectifier provides distortion on the load side electronics. This in turn can create voltage distortion if there is a significant output impedance of the EMI filter at the current distortion frequencies. To minimize the EMI filter interaction, the passband of the EMI filter is traditionally designed to be wider than would be optimal. The design process then becomes a tradeoff between the two opposing performance criteria of either optimizing EMI suppression or minimizing harmonic current distortion.
Accordingly, there is a need in the technology for an apparatus and method of providing EMI filtering in electronic units implementing power factor correction circuitry, which provides both optimization of EMI filtering and minimization of harmonic current distortion. There is also a need in the technology to provide a sense circuit that is position independent with respect to the main rectifier so that the performance of the PFC portion of the electronic unit may be enhanced.